Ring-opening polymerization of a galla[1]ferrocenophane: A gallium-bridged polyferrocene with observable tacticity

Article abstract of DOI:10.1021/ja910648k

(Figure Presented) A gallium-bridged polyferrocene was prepared via spontaneous ring-opening polymerization of a galla[1]ferrocenophane. This organometallic polymer is thermally robust and can be purified and handled under ambient conditions, making it an

Full text of DOI:10.1021/ja910648k

Published on Web 01/25/2010
Ring-Opening Polymerization of a Galla[1]ferrocenophane: A Gallium-Bridged
Polyferrocene with Observable Tacticity
Bidraha Bagh,† Joe B. Gilroy,‡ Anne Staubitz,‡ and Jens Mu¨ller*,†
Department of Chemistry, UniVersity of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan S7N 5C9, Canada,
and School of Chemistry, UniVersity of Bristol, Bristol BS8 1TS, United Kingdom
Received December 17, 2009; E-mail: jens.mueller@usask.ca
Since Manners et al. reported that thermal ring-opening poly-
merization (ROP) of dimethylsila[1]ferrocenophane (1) yields high-
molecular-weight poly(ferrocenyldimethylsilane) (2),1a the tool box
for ROP of strained sandwich compounds has been developed
significantly.1 To date, in addition to thermal ROP, transition metal
catalyzed-, anionic-, and photocontrolled ROP of metallacyclo-
phanes are described in the literature.1c The latter two methods are
of particular interest as they can be performed as living polymeriza-
tions, which give access to block copolymers.
allacyclophanes as the only isolatable products [3 (M ) Fe) and 4
(M ) Cr, Mo)].11 Obviously, the bulkiness of the group-13 bound
ligand has a major effect on the outcome of reactions between a
dilithiated sandwich species and an aluminum or gallium dihalide.
Within this paper, we report on results obtained using a new “one-
armed phenyl” ligand designed to incorporate steric bulk.
Starting from commercially available 3,5-di-tert-butyl-toluene,
the known amine 512 was prepared in three steps (eq 1), from which
the gallium dichloride 6 was obtained as an analytically pure solid
(see Supporting Information (SI) for details). Species 6 reacted
readily with dilithioferrocene to form the targeted galla[1]ferroceno-
phane 7 as an intermediate (Scheme 1).
Poly(ferrocenylsilanes) are functional materials applicable as
plasma-etch resists for nanopatterning,2 precursors to ceramics,3 a
tunable component of photonic crystals displays (photonic ink),4
redox-tunable surfaces,5 and polyelectrolyte capsules with redox-
dependable permeability.6 Block copolymers in block-selective
solvents allow control of different micelle morphologies, and
poly(ferrocenyldimethylsilane) (2) containing block copolymers,
which form cylindrical micelles with semicrystalline cores,7 show
significant promise for future applications in nanotechnology.8
Despite these recent advances, the number of well-defined
metallopolymers is still quite restricted. Recently, we synthesized
strained sandwich compounds with aluminum or gallium in bridging
positions with the aim of developing new polymeric materials
through ROP.9 In this paper, we describe the first, well-characterized
polyferrocene with gallium in bridging positions.10 In depth NMR
spectroscopy studies reveal that this air-stable organometallic
polymer shows a surprising sensitivity toward the stereochemistry
of the polymer backbone.
Scheme 1. Synthesis of Intermediate 7 and Polymer 7n
Attempts to isolate this new strained sandwich compound 7 gave
an orange powder, which was characterized as the poly(ferroce-
1
nylgallane) 7n. The formation of species 7 was confirmed by H
NMR spectroscopy studies. If the Et2O from an aliquot of the
reaction mixture, taken after ca. 15 min, was quickly replaced by
C6D6, intermediate 7 was observed by 1H NMR spectroscopy with
resonances in the typical Cp range at δ 4.69 (4 ꢀ-H), 4.56 (2 R-H),
and 4.01 (2 R-H). This pattern and the chemical shifts match very
well with other gallium-bridged [1]ferrocenophanes we have
characterized previously [bridging moiety Ga(Pytsi):9b δ 4.65 (2
ꢀ-H), 4.61 (2 ꢀ-H), 4.45 (2 R-H), 4.08 (2 R-H); or Ga(Me2Ntsi):9c
δ 4.54 (4 ꢀ-H), 4.24 (2 R-H), 3.90 (2 R-H)].
Polymer 7n was purified by precipitation into MeOH (45%
isolated yield) and characterized by elemental analysis, GPC, DLS,
All known strained Al- or Ga-bridged [1]metallacyclophanes9
were equipped with the bulky, intramolecularly coordinating, trisyl-
based ligands Pytsi or Me2Ntsi. However, ROP attempts with these
[1]ferrocenophanes and [1]ruthenocenophanes either failed or
resulted in sluggish polymerizations.9e On the other hand, employ-
ing the “one-armed phenyl” ligand Ar′ or p-tBuAr′ gave [1.1]met-
1
WAXS, DSC, TGA, CV, UV/vis, H and 13C NMR spectroscopy
(SI). Material 7n is an amorphous polymer with a glass transition
at 205 °C. The polymer is thermally robust as it retained 98% of
its mass at 340 °C; further heating to 600 °C gave a nonmagnetic
char with a low yield of 14% (TGA). Electrochemical studies (cyclic
voltammetry) revealed two poorly resolved oxidation waves and
one broad reduction wave. The midpoint between the main redox
† University of Saskatchewan.
‡ University of Bristol.
+
waves was found at -0.047 V versus the couple FeCp2/FeCp2
.
9
1794 J. AM. CHEM. SOC. 2010, 132, 1794–1795
10.1021/ja910648k  2010 American Chemical Society

C O M M U N I C A T I O N S
GPC analysis showed a broad peak corresponding to an Mw of 48
kDa (DPw ) 96, PDI ) 3.3) with respect to polystyrene. Dynamic
light scattering of 7n resulted in a hydrodynamic radius of 2.99 ( 0.36
nm. Assuming that 7n can be described as a random coil, with CH2Cl2
being a good solvent, this Rh value gives a radius of gyration of 6.13
( 0.74 nm (Rg/Rh ) 2.05),13 which translates into an Mw of 36 kDa
(DPw ) 72) with respect to poly(ferrocenyldimethylsilane).14
1H NMR spectra of 7n showed broad peaks relative to those of
the monomer 7, with some peaks exhibiting a rich fine structure.
For example, the tBu group para to gallium appears as a singlet (δ
1.32), whereas the tBu group ortho to gallium is split into 10 signals
(centered at δ 1.53). This ortho-tBu group is oriented toward the
polymer backbone and acts as a fine sensor for the tacticity of 7n.
As shown in Figure 1, the 10 peaks can be sorted into three groups
with 3, 4, and 3 singlets, respectively (see A, B, and C in Figure
1). Every Ga atom in polymer 7n is a stereogenic center, and
neighboring Ga atoms could have either the same or a different
chirality, leading to racemo or meso diads. For three repeat units,
three different arrangements are possible, which are illustrated in
Figure 1. The splitting of the signal of the ortho-tBu group into
the three groups A, B, and C is due to a triad sensitivity. The
approximate intensity ratio between those three groups is 1:2:1,
indicating that 7n is a polymer with a statistical distribution of
stereogenic centers; hence, signal B can be assigned to heterotactic
triads mr and rm. The fine structure in A, B, and C can be explained
by a further sensitivity toward pentads.
spontaneous ROP results in 7n. This behavior is reminiscent of the
chemistry of phospha- and stanna[1]ferrocenophanes.1b,c,16 Interest-
ingly, one of the two tBu groups of the ligand points toward the
polymer backbone and serves as a very sensitive probe of the
polymer stereochemistry.17 Polymer 7n is thermally robust and can
be purified and handled under ambient conditions, making it an
ideal candidate for incorporation into polymer based materials
offering an alternative to existing polyferrocenes. Future work will
focus on the isolation of monomers such as 7, toward the realization
of well-defined poly(ferrocenylgallanes) via living ROP methodologies.
Acknowledgment. Dedicated to Prof. Peter Paetzold (RWTH
Aachen) on the occasion of his 75th birthday. We thank Prof. Ian
Manners (University of Bristol) for sabbatical support and helpful
discussions and NSERC of Canada (J.B.G. for a PDF; J.M. for a
DG) and EPSRC of the United Kingdom (A.S.) for support.
Supporting Information Available: Experimental section for 5, 6,
1
and 7n; H NMR spectrum of 7; spectra and diagrams for 7n (NMR,
GPC, DLS, WAXS, DSC, TGA, CV). This material is available free
of charge via the Internet at http://pubs.acs.org.
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Figure 1. Illustration of different triads in 7n (the NMe2 group at the aryl
ligand is omitted for clarity). 1H NMR signal of the ortho-tBu group of 7n
exhibiting pentad resolution (intensity ratio A/B/C ≈ 1:2:1).
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An extension from three to five repeat units results in 16 possible
pentads, from which 10 are distinguishable.15 In case of a polymer
with a statistical distribution of stereogenic centers, isotactic and
syndiotactic triads both result in four pentads from which three are
distinguishable giving a distribution ratio of 1:2:1 (A and C in
Figure 1). In contrast, the heterotactic triad results in eight pentads
with four being distinguishable giving a distribution ratio of 2:2:
2:2 (B in Figure 1). Expectedly, the signal pattern of the ferrocene
units is more complex than that of the ortho-tBu group. The Cp
protons could show a diad or tetrad sensitivity, with the latter being
the pendant to the pentad sensitivity of the tBu group. A diad
sensitivity in the form of two sets of four Cp signals in an
(12) Yoshifuji, M.; Kamijo, K.; Toyota, K. Tetrahedron Lett. 1994, 35, 3971–
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1
approximately 1:1 intensity ratio can be clearly seen in H NMR
(13) Burchard, W. AdV. Polym. Sci. 1999, 143, 113–194.
spectra of 7n. Some of the individual peaks show an additional fine
structure, but a full resolution into tetrads was not observed. The
13C{1H} NMR spectrum of 7n is similarly complex as the proton
spectrum. For example, while the tertiary carbon atom of the para-
tBu group gives one resonance only, that of the ortho-tBu group
gives three signals (triad resolution).
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(15) The splitting of triads into 16 pentads can be illustrated as follows
(indistinguishable pentads shown in parentheses): isotactic triad: mmmm,
(mmmr, rmmm), rmmr; heterotactic triad: (mmrm, mrmm), (mmrr, rrmm),
(rmrm, mrmr), (rmrr, rrmr); syndiotactic triad: mrrm, (mrrr, rrrm), rrrr.
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1805–1807.
In summary, all attempts to synthesize the strained [1]ferro-
(17) Poly(ferrocenylmethylphenylsilane) showing a triad resolution is described
in: Rasburn, J.; Foucher, D. A.; Reynolds, W. F.; Manners, I.; Vancso,
G. J. Chem. Commun. 1998, 843–844.
cenophane 7 resulted in the isolation of the air-stable poly(ferro-
1
cenylgallane) 7n. However, H NMR spectroscopy revealed that
the targeted monomer is first formed and one can assume that a
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